A conventional semiconductor package used as an image sensor device typically includes a lead frame having a die pad and a plurality of leads, and an encapsulant formed with the lead frame. The encapsulant is formed with a cavity for receiving a semiconductor chip mounted on the die pad that is exposed to the cavity. In addition, a lid is adhered to the encapsulant in such a manner that the cavity is sealed thereby, allowing the semiconductor chip to be separated from the ambient. During the formation of the encapsulant, a molding resin for forming the encapsulant usually flashes on the portion of the lead frame exposed to the cavity, which thus contaminates surfaces of the die pad and leads that are exposed to the cavity, so that the die bonding between the die pad and the semiconductor chip, and wire bonding between the semiconductor chip and the leads, are adversely affected.
Accordingly, U.S. Pat. No. 5,070,041 discloses a method of removing flashes from a semiconductor lead frame, in which prior to forming an encapsulant on the lead frame by a molding resin, the lead frame is coated with an organic high molecular substance on its portions expected to be in non-contact with the molding resin. Such an organic high molecular substance has a melting or softening point higher than the molding temperature of the molding resin for forming the encapsulant, and is soluble in a solvent which does not dissolve the encapsulant. Thereby after a molding process is completed with the formation of the encapsulant, the coated lead frame bearing the encapsulant is immersed in the solvent for dissolving away the coated organic high molecular substance. As a result, undesired flashes of the molding resin occurring on the lead frame can be removed in facility with the dissolution of the organic high molecular substance, so as to assure good electrical connection between the lead frame and a semiconductor chip mounted on the lead frame.
However, the foregoing flash-removing method in the use of the organic high molecular substance is rather complex in proceeding with a procedure of coating the lead frame with the organic high molecular substance, and an additional post-treatment of using the solvent for dissolving away the coated organic high molecular substance, thereby making fabrication processes undesirably increased in cost and time consumption but decreased in productivity and efficiency. Therefore, how to develop a more simplified method for fabricating a semiconductor package in prevention of resin flashes is highly desired, which can be more cost-effectively implemented, and effectively help eliminate the occurrence of resin flashes as well as improve package productivity.